Bottom-up and top-down controls of walleye pollock in the Eastern Bering Sea Franz J. Mueter Sigma Plus, Fairbanks, Alaska, USA fmueter@alaska.net Brenda L. Norcross Michael C. Palmer IMS, SFOS, University of Alaska
Bottom-up control Recruitment limited by primary production (environmental variability) Ice extent / timing of retreat Cold pool extent Surface currents( larval transport) Wind mixing / Stratification
Ice extent / timing of retreat Stock-recruit residual -1.0 0.0 0.5 1.0-1.0 0.0 0.5 1.0 R 2 = 0.11 (p = 0.064) R 2 = 0.28 (p = 0.002) R 2 = 0.16 (p = 0.023) 20 4012 14 60 16 18 20 22 24 Sea ice extent Week of ice retreat
Fate of primary production Early ice retreat Late bloom Warm SST High pelagic prod. Late ice retreat Early (ice-edge) bloom Cold SST High benthic prod. Stock-recruit residual (standardized) -2-1 0 1 2 pollock yellowfin sole walleye pollock yellowfin sole
Yellowfin sole and walleye pollock Yellowfin sole stock-recruit residual -0.5 0.0 0.5 1.0 r = -0.63 (p < 0.001) -1.0-0.5 0.0 0.5 1.0 Walleye pollock stock-recruit residual
Cold Pool extent 1992 Alaska > 2 0 C < 2 0 C Aleutians
Cold Pool extent Index of association modified and updated from Wespestad et al. (2000) Stock-recruit residual -1.0 0.0 0.5 1.0 Index of association 1.0 1.5 2.0 2.5 Smoothing spline with 95 R 2 = 0.07 95% confidence bands 99 98 88 94 87 96 01 82 02 90 84 92 89 00 91 86 03 97 93 (p = 0.16) 83 85-1 0 1-1 2 0 1 2 Cold Pool Index Cold Pool Index
Surface transport Center of Spawning From: Wespestad et al. (2000). ICES J. Mar. Sci 57:272-278
Surface transport Stock-recruit residual -1.0 0.0 0.5 1.0 R 2 = 0.11 (p = 0.046) 50 52 54 56 58 60 Final latitude of larval drift (OSCURS trajectories)
Summer stratification R 2 = 0.38, p =0.002 Partial SR residual -1.5 - -1.0 - -0.5-0.0 0.5 1.0 130 140 150 160 170 180 190 Onset of stratification (Julian day) (from Carol Ladd, PMEL, pers. comm.) -1.5 - - -1.0 - -0.5 0.0 0.5 1.0 300 350 400 450 500 550 600 May-Sep wind stress (m 3 /s 3 ) (St. Paul Island) Summer conditions best explained past variability in survival rates of walleye pollock
Best overall model: Retrospective analysis log(r t /S t ) = b 0 + b 1 * SSB t-2 + b 2 * Strat t-1 + b 3 * (Wind t-1 ) 2 + b 4 * Lat t-2 + ε τ log(recruits / SSB) 0 1 2 3 Out-of-sample predictions 1970 1980 1990 2000 Year class
Summary: Bottom-up controls Ice extent / timing of ice retreat Fate of primary production Oscillating Control Hypothesis Cold pool extent Cold pool hypothesis Surface transport Larval transport hypothesis Summer stratification / wind mixing New hypothesis??
Match / mismatch hypothesis Match Mismatch Stock-recruit residual -1.5 - - -1.0 - -0.5 0.0 0.5 1.0 130 140 150 160 170 180 190 Onset of stratification (Julian day) Stratification index from Carol Ladd (pers. comm.) Aboveaverage survival Belowaverage survival
Top-down control Recruitment limited by predation from top-predators Predation Cannibalism
Top-down control: Consumption of walleye pollock Total consumption of walleye pollock as high as 3.5 million t (Livingston & Lang 1996) Model estimates based on diet composition ~40% of juvenile production (age 0-1) consumed by adult pollock (age 2+) ~5% of the diet of age 2+ pollock consists of age 0-1 pollock (Kerim Aydin, pers. comm.)
Cannibalism: Stock-recruit relationship Strong density-dependence in spawnerrecruit relationship Age-2 log(recruits Recruits (billions) / SSB) 0 10 1 20 30 2 40 3 50 60 4 0 1000 2000 3000 4000 Spawning Stock Biomass (1000 t)
Predation mortality Predation mortality accounts for 33% of remaining variability in log-survival (1979-2001) Or as much as 76% of total variability in logsurvival Stock-recruit residual -1.0 0.0 0.5 1.0 log(recruits / SSB) 0 1 2 3 R 2 = 33% (p = 0.004) R 2 = 76% (p < 0.001) 0.8 1.0 1.2 1.4 0.8 1.6 1.0 1.8 1.2 1.4 1.6 1.8 Age-1 predation mortality Age-1 predation mortality Predation estimates from Jurado-Molina and Livingston (pers. comm.)
Top-down control Predation mortality depends on Abundance of predators Primarily adult pollock Availability of juveniles to predators Spatial overlap: horizontal / vertical Age-1 mortality 1.2 1.4 1.6 1.8 1.0 1.5 2.0 2.5 Index of association
Top-down control Index of spatial association as predictor of log-survival and recruitment observed predicted Stock-recruit residual -1.0-0.5 0.0 0.5 1.0 R 2 = 0.304 1.0 1.5 2.0 2.5 Index of association log(recruitment) 7.5 8.0 8.5 9.0 9.5 10.0 1985 1990 1995 2000 Year class
Bottom-up and top-down controls Other predators Environmental control? Predation (age-0) (age-1) Prey availability Timing of summer bloom Wind mixing Fate of primary production Ice retreat
Special thanks to: Carol Ladd, PMEL Nick Bond, PMEL Dave Kachel, PMEL Jim Ianelli, AFSC Pat Livingston, AFSC